I need some help! Carb/misfire

MILLENNIUM FALCON

MILLENNIUM FALCON

EL Guapo
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This bike is an 81 xs400 special. I also posted this in the build section but I dont think anyone is going to read it. Here is the low down. I am having some fuel issues. I went over everything today. The carbs are clean and all parts pass visual inspection. I have good spark, and the plugs look fine. I had a bad miss on the left cylinder. It had spark but wasn't getting fuel. I know this because the plug was cold and dry, The header was about 100 degrees cooler (inferred temp gun) than the right header,I could pull the left plug wire off while it was running and nothing would change, but if I squirted some carb cleaner in the left carb it would pick up and smooth right out. the carb bowl is full of gas but it doesnt seem to be getting into the intake. I pulled the carb again, but I couldn't find any problems. Now when I start it up it seems to be firing on both cylinders, (most of the time) header temps are much closer, but now it wont idle. In the video I start the bike and it climbs up to 4500rpm, when I blip the throttle it climbs but wont go below 4500. When I pull the choke out it goes down to 1200rpm. Can anyone give me any direction on this? Thanks.

Update:

Ok now i'm mad. $30 on cans and buckets of cleaner. Soaked the carbs all night, meticulously cleaned and reassembled. Now when I run it, it idles ok, but is still running on one cylinder. (Same as before) So, im thinking maybe its not fuel. I am noticing that The plug wire is shorts out to my finger when I touch it, so I thought maybe it just a bad plug wire this whole time? So I out on a spare plug wire I have from my parts bike, and it is still shorting out to my finger. I electrical taped the entire wire plugs, ends and at the coil. IT IS STILL jumping to whatever is around it. Is my coil leaking or something? Now i'm thinking its a spark problem. Im going out to switch the plug wire from the side I know is working. Any clue??????????????? I'll be back in a few..

Update:

ell that was a waste of time. I swapped wires and coils....no change. I cannot get the left cylinder to fire. I checked the valves and compression and it passes. I changed plugs too. I have a very strong spark. Now when I start it, it idles ok but If I touch the throttle it revs really high and just stays there (5000) I checked to make sure it wasnt the throttle cable, its not.
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Do you have points or an electronic setup on that '81?

I need a clearer picture of this "shorting out" of the plug wire. Is it jumping to your finger while completely installed on a spark plug, or is it jumping when you pull it off? Where is it Jumping from? Is it the metal contact that touches the spark plug or somewhere along the length of the wire?

Does it do this on both sides? If you swapped coils and the behavior remains it is probably normal. It should jump to you or another metal surface, but not through the sides of the cable or from the coil body itself.

If you've swapped coils and there is no change, it isn't the coils or ignition. If compression is also good the internals are probably just fine. This leaves fuel delivery or vacuum leaks. Since one side is running fine everything from the gas tank to the carb body is likely fine. This leaves the carbs, especially the carb on the side that wont fire, or a vacuum leak in the carb/intake/exhaust assembly.

Try using propane gas to find the leak. While the bike idles spray some gas around the intake/carbs. If the idle seems to change you've probably found a leak. If this doesn't work your carbs are the culprit. Leaking o-rings, or improper float height, idle fuel screw settings, etc.
 
This bike has electronic ignition. I got 150 psi comp. On both sides.
I finally got the bike running on both cylinder! but I still cant get it to idle. It runs up to 5000 rpm as soon as I hit the starter button. The weird thing is the butterfly's are closed and the slides are all the way down. How is that possible? I tried spraying carb cleaner on everything when it was actually idling (1100rpm) to check for vacuum leaks, but it did not change the engine speed.* I also used a propane torch. It seems like it has to be getting air from somewhere. Dont ya think?
 
The "shorting out" problem doesn't seem to be the issue anymore. It was jumping from the plug wire to the cylinder head or my finger if I touched the wire while it was running.
 
That jumping is normal; that is how I check for spark. I generally turn the engine over by hand though as I only get one shock instead of a dozen :)

Ok, it seems like you're running way too lean. You said the idle DROPS when you pull the choke out? Since choke richens the mixture greatly it normally causes your idle to shoot way up above normal. If choke is bringing it back down to normal you're too lean.

If you can't seem to find a leak with gas it must be the carbs. What kind of intake/exhaust setup are you running? Is the bike stock? I can't see what you're running in the video.

How do you know the butterflys are closed? Have you tried adjusting the throttle cable tension?
 
The bike is stock. The choke doesn't do a thing now. It was bringing down the idle, but now. It has no effect. Are there any common problems with these carbs. I know the butterflies are closed becaus I disconnected the throttle cable to eliminate binding. I also held it to the idle stop to make sure it wasnt creeping. I have worked on alot bikes Before but never had this much trouble. I'm sure it's something simple but it is frustraighting. I have visually inspected the diaphragms they look really good but is there "best" way to check them? They fall down smooth and steady
 
If you can't see any holes and they fall smoothly the diaphragms are probably just fine. The carbs on these bikes are crabby and need to be cleaner than you'd think. Additionally, the manuals often have incorrect values for float heights and such.

How many turns out are you on the idle mixture screws? What is your float height set at? Do you have brass or rubber/plastic floats?
 
I have brass floats set at 26mm the mix screw is at 3.5 turns from stop. My rubber plugs that cover the slow jet are pretty stiff and don't look the best. Could this be a problem? Everything else looks it good shape. I found a bunch of threads on here about what to check to make sure the carbs are properly cleaned. Everything is clear. Thanks for helpping me out.
 
MILLENNIUM FALCON said:
I have brass floats set at 26mm the mix screw is at 3.5 turns from stop. My rubber plugs that cover the slow jet are pretty stiff and don't look the best. Could this be a problem? Everything else looks it good shape. I found a bunch of threads on here about what to check to make sure the carbs are properly cleaned. Everything is clear. Thanks for helpping me out.
Click to expand...

I think that may be your problem. Get some proper Fuel resistant O rings from mike xs, if fact you can get the kit with needle and replace it all!
 
Are you talking about the needle seat o-ring because I did replace them. I am talking about the rubber plug that hits the carb bowl and covers the passage to the slow jet. Thanks
 
edit: I didn't see that you have replaced the o-rings.

Have you tried adjusting the synch screw on the carbs? Can you feel a difference in exhaust pressure at idle on each side? I'm just trying to throw more ideas out there.
 
Yeah I replaced the small o-rings on the float needle seat. The one with the sceen filter on it. They were completely flat the brass piece was loose in the hole.
 
Oh! Your model might have o-rings under the idle mixture screws. I am not sure what year this started, but I am pretty sure the carbs with rubber bits have them. Often they disintegrate! I have heard of people not being able to find them at all, but who knows if the previous owner removed them.

Since you're 3.5 turns out you must not have any temper proof caps anymore. If that is the case, someone must have removed the screws before.

I think these pieces even get omitted from manuals and diagrams because Yamaha didn't want the public to have access to emissions-related parts.
 
Drewpy, xschris, or Houghmade, know the different years better than me. All I can say is that my 1978 carbs have all brass parts and no rubber aside from the choke plungers.

You are probably supposed to have them, but someone else can confirm. If they are missing or damaged the bike will definitely not idle right.
 
I have looked online for a good diagram of my carbs to make sure nothing is missing. The previous owner had the carbs apart so who know what he did. I only know what I see. I have a manual in the mail hopefully it has some carb pictures
 
well that could be my problem. It says the plugs were to keep people from tampering with the screw, but it also said the bike wont run right without it in place. My bike has nothing covering the screw hole. I will plug them and try running it again. It also said they should be adjusted from 2.5-3.0 turns out not 3.5 turns. ill let you know what happens.
 
The blind plug won't keep it from starting, but the lack of an o-ring will. The blind plug just keeps debris out if anything, but you don't need it. The 2.5 - 3 turns out thing is just a guide; 3.5 turns out is reasonable and if it works don't worry about it. What is most important is whether you can see an o-ring and washer in there below the screw.
 

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